1. Field of the Invention
The present invention relates to a CMOS current mirror.
2. Description of the Background Art
A type of CMOS current mirror is known, for example, from Tietze/Schenk, “Halbleiterschaltungstechnik” (Semiconductor Technology), ISBN 3-540-19475-4, 9th ed., Springer-Verlag, Berlin/Heidelberg/New York, pp. 96 and 97. A current mirror with a resistor between gate terminals of an input and output transistor is known furthermore from the publication “A Novel Highspeed Current Mirror Compensation Technique and Application”, Thart Fah Voo, Toumazou, C., IEEE International Symposium on Circuits and Systems; 1995, Vol. 3, 28 Apr. to 3 May 1995, pages: 2108 to 2111. Furthermore, current mirrors with cascode transistors for increasing the output resistance are conventional in the art. Various CMOS current mirrors are also disclosed in U.S. Patent Application 2004/0056708 A1.
The principle of action of a CMOS current mirror is based on the operation of the input transistor and the output transistor with same gate-source voltage in the saturation region. If both transistors are identical, the same currents flow in their conductivity paths. In this regard, a conductivity path is understood to be a current path connecting the drain and source of a MOS transistor with inclusion of channel regions and optionally present drift regions. If transistors with different transistor geometries are used in the input branch and output branch, the quotient w2*l1/l2*w1 determines the quotient of the output current strength and input current strength, which is also called the current transformation ratio. Here, w indicates the channel width, l the channel length, the subscript 1 the input transistor, and the subscript 2 the output transistor. In addition, current mirrors permit generation of integer multiples or fractions of the input current by parallel connection of a suitable number of identical transistors to the output transistor or the input transistor.
Changes in the input current do not arise without a delay, but with a certain delay in the output current, which depend on the transconductances gm of the transistors, therefore on the quotients of the drain currents in the numerator and gate-source voltages in the denominator, and on the gate-source capacitance of the transistors. This delay is troublesome for some applications. High rates of change in the output current are required, for example, in high-speed current DACs (DAC=digital analysis converter) and laser drivers in CD and/or DVD devices during rapid writing processes. The invention is not limited to such applications, however. Rather, DACs have found wide use, so that the invention can be used wherever such DACs have a current output and must be reasonably fast.
The following list gives a few examples, whereby the enumeration is not definitive: low cost measuring technology, programmable voltage sources and current sources, measuring technology in the automotive sector, precision motion control as in the printing industry, regulators in the automotive sector, digital programmable current loops, as they are used in telecommunications, programmable logic controllers, input/output cards, mobile telephones, high-speed digital/analog testers, pagers, fiber optic switching exchanges, power amplifiers, and control of voltage-controlled oscillators (VCO control). Current mirrors are also used otherwise in many different ways in the design of integrated circuits, for example, for supplying current to circuit parts such as amplifiers or mixers, for analog signal processing, or as interfaces between two circuits, because transmission of currents is less sensitive than transmission of voltages to interference in a reference potential.
In the aforementioned conventional current mirror, which has an ohmic resistor connected between the gate terminals of the input and output transistors, the rise and response time of the output current after a change in the input current depends on the value of the resistance and on the current. In this case, the optimal resistance value changes with the current and therefore must be constantly adapted.